1. Field of the Invention
This invention relates generally to microwave transmission equipment and, more particularly, to attenuator (equalizer) units which alter the signal attenuation frequency response of the transmission lines, traveling wave tubes, and microwave systems.
2. Description of the Related Art
The use of attenuator units in microwave applications for controlling the frequency response of microwave components is well known. In U.S. Pat. No. 3,648,200, entitled Frequency Selective Attenuation Apparatus, issued on Mar. 17, 1969 to W. H. Harrison et al., and U.S. Pat. No. 4,117,425 issued to H. Bacher on Sep. 26, 1978, an attenuator unit including a pair of tuning stubs, spaced approximately a quarter wavelength apart, is disclosed. The attenuation characteristic of the attenuation unit can be determined by the frequency of the incident energy and by the length of the tuning stubs. The impedance changes induced by one stub receive partial compensation by positioning the second stub at one quarter wavelength distance from the first stub. The attenuator unit configuration described by the referenced U.S. patents has two primary disadvantages. First, the required quarter wave spacing between the stubs limits the utility of the attenuator unit to a relatively narrow frequency range. Second, at frequencies other than the frequency for which the distance between the tuning stubs is a quarter wave, substantial energy is reflected back toward the signal source rather than being dissipated by the attenuator unit. In addition, and partially as a result of limiting the frequency range to a region determined by the spacing of the stubs, signals propagated along the transmission line have a frequency dependent delay. Devices used to fine tune frequency response of the attenuator unit present severe difficulties that require tedious manual adjustment to achieve the desired frequency response and, once achieved, the frequency response is unstable with temperature variations.
A need has therefore been felt to provide an attenuator unit which exhibits a nearly constant input and output (i.e., matched) impedance and exhibits a nearly constant delay over a very wide frequency range. A need has also been felt to provide manual adjustments for which the frequency response remains constant with temperature.